Heat and Mass Transfer
Thermal diffusivity is a

All of these
Dimensionless parameter
Physical property of a substance
Function of temperature

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Heat and Mass Transfer
According to Prevost theory of heat exchange

All bodies above absolute zero emit radiation
It is impossible to transfer heat from low temperature source to t high temperature source
Heat transfer by radiation requires no medium
Heat transfer in most of the cases takes place by combination of conduction, convection and radiation

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Heat and Mass Transfer
A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,

The better insulation must be put inside
One could place either insulation on either side
One should take into account the steam temperature before deciding as to which insulation is put where
The better insulation must be put outside

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